I. Field of the Invention
The invention relates to a method for connecting a buoy to a floating vessel, for transfer of a flowable medium to or from the vessel, wherein the vessel is brought into position above a submerged loading/unloading buoy which is anchored to the sea bed and is connected to at least one transfer line for medium, and a hoisting means on the vessel is connected to the buoy whereafter it is hoisted up and inserted into a downwardly open submerged receiving space in the vessel.
Further, the invention relates to a system for connecting a buoy to a floating vessel, for transfer of a flowable medium to or from the vessel, comprising a submerged loading/unloading buoy which is anchored to the sea bed via catenary mooring lines and is connected to at least one transfer line for medium, a downwardly open submerged receiving space arranged on the vessel for receipt of the buoy, and a hoisting means arranged on the vessel for connection with and hoisting of the buoy, so that it is inserted into the receiving space.
II. Description of Related Art
A method and a system of the above-mentioned type are known from e.g. U.S. Pat. specification No. 4,604,961 (corresponds to Norwegian patent specification No. 167 906). This known system is based on a vessel having a through-going deck opening in a central region of the vessel, the lower part of the through opening forming the submerged receiving space for a mooring element in the form of a submerged buoy. In the receiving space there is arranged a rotating body (turret) which is rotatably mounted in the hull of the vessel and is designed for receipt and attachment of the mooring element, the latter to this end being provided with a hydraulically actuated locking mechanism for attachment to the rotating body. Further, the vessel is provided with a derrick for the lowering of a retrieval string having a retrieval connector at its lower end for interconnection with the mooring element, so that this may be pulled up and into the receiving space. The interconnection is obtained in that the mooring element is provided with a conical centering receptacle having a socket arranged at the bottom wherein the retrieval connector may be received and secured, e.g. by means of a bayonet lock. The lower end of the retrieval string preferably is provided with sonar and TV equipment to ensure positioning of the retrieval connector in the centering receptacle.
The known system is encumbered with some drawbacks which will be discussed below.
As mentioned, the vessel of the known system is based on a through-going deck opening, which reduces the strength of the vessel and poses demands for additional reinforcements in the bottom and the deck of the vessel. Experience has also shown that ships having a through-going deck opening are subject to fatigue in the hull.
Since the rotating body is attached to the vessel under water, this requires divers for inspection and minor maintenance. Major maintenance requires docking of the vessel. Because of the fact that the rotating body is mounted to the vessel, there arise large frictional forces which are to be overcome by torques from the mooring element. These torques are relatively large due to the large outer diameter of the rotating body, and this results in correspondingly large loads. Further, it may result in uncontrolled rotation of the system, so that it becomes necessary to use a braking system for retaining the rotating body. In case of rotation the braking system is then released and the rotating body is rotated in a controlled manner by means of active drive.
The known system has a small ability to absorb moments caused by the horizontal mooring forces, something which results in a substantial risk for jamming actions in the mounting arrangement.
The hydraulically actuated locking mechanism which is arranged on the mooring element requires divers for connection of the control hydraulics. Diver operations in connection with connection and disconnection render the use of the system as a transport system impossible, when using shuttle tankers. Further, there is a big risk for faulty operation and damages in case of uncontrolled disconnection. In case of breakage of the hydraulic system there is no possibility for the connection of a back-up or auxiliary device.
As mentioned, connection/disconnection takes place by means of a derrick-operated string having a special retrieval means. When connecting, this requires small relative movements between vessel and mooring element/buoy, so that the connection can be carried out in a safe manner only under relatively calm weather conditions. Also this circumstance makes the system unusable as a transport system with shuttle tankers. Further, the connecting as well as the disconnecting operation requires a relatively long time to be accomplished.